MainStage is primarily designed for a single performer, but can work for multiple keyboards. Within a single patch each connected keyboard can be connected to one or more instrument channel strips (layers/splits). For example, some more advanced setups could include the classic 3 keyboard setup where one player has a keyboard in front and one on each side all playing different sounds inside of a single patch. If there are several players, it would be ideal for each of them to manage their own sounds/patches.
As for sounds, MainStage comes with "Patches" which can contain one or more channel strip. There are lots of patches designed to work with the included templates concerts. These all include on screen controls mapped to plug-in parameters. The non-mapped patches appear to provide sounds, but leave it to the player to decide which plug-in parameters (if any) they want to map into the screen controls.
Beyond that, there are probably thousands of software instrument channel strip settings that can be loaded in the SI channel strips inside a patch. Basically you get the entire 50GB sound library at your disposal for sounds. You can even play Apple Loops in the Playback plug-in, or use it for backing tracks you make in Logic (or elsewhere).
If you plan to use 3 keyboard rigs, and don't yet have computers, you should consider the MacBook Air which is small, lightweight and fairly inexpensive. The 13" MacBook would work very well too.
Keep in mind that its a good idea to use plenty of RAM. Always construct your patches to make sure you don't exceed available CPU. The other thing to consider is that MainStage, by default uses something they call "seamless patch switching" which means that if you are playing patch A, and hold a chord, then switch to patch B, patch A will continue to sound until you issue new notes. This means that you don't "cutoff" the previous patch during a patch change. Once patch A is no longer playing notes, it will stop using CPU. This means that you need enough CPU to play two patches at once. I only mention this because the feature is really cool, but caught me off guard before I understood how it worked.
Using Logic is certainly possible, but its not really a live performance app. It has no "patch" concept which turns out to be very useful for live performance. Also, the interface is a bit much during live performance. The full screen view of MainStage is a refreshingly simple way to work with your sounds. You don't have to look at plug-in UIs or small mixer UIs, etc. Basically, you can focus only on the things you require for your set. In my case, this is usually very little info, like song name, patch name, etc. Sometimes I use some knobs, but usually its just switching between sounds.